A fusion process (e.g., downdraw process) forms high quality thin glass sheets that can be used in a variety of devices such as flat panel displays. Glass sheets produced in a fusion process have surfaces with superior flatness and smoothness when compared to glass sheets produced by other methods. The fusion process is described below with respect to FIG. 1 (Prior Art) but for a more detailed description refer to co-assigned U.S. Pat. Nos. 3,338,696 and 3,682,609, which are incorporated herein by reference in their entireties.
FIG. 1 shows a schematic view of an exemplary glass manufacturing system 10 which utilizes the fusion process to make a glass sheet 12. As shown, the exemplary glass manufacturing system includes melting vessel 14, fining vessel 16, mixing vessel 18, delivery vessel 20, fusion draw machine (FDM) 22, and traveling anvil machine (TAM) 24. Typically, components 16, 18 and 20 are made from platinum or platinum-containing metals, but they may also comprise other temperature resistant metals.
Melting vessel 14 is where the glass batch materials are introduced as shown by arrow 26 and melted to form molten glass 28. Melting vessel 14 is connected to fining vessel 16 by melting to fining vessel connecting tube 30. Fining vessel 16 has a high temperature processing area that receives molten glass 28 (not shown at this point) from melting vessel 14 and in which bubbles are removed from molten glass 28. Fining vessel 16 is connected to mixing vessel 18 by a finer to stir chamber connecting tube 32. Mixing vessel 18 is connected to delivery vessel 20 by a stir chamber to bowl connecting tube 34. Delivery vessel 20 delivers molten glass 28 through a downcomer 36 into FDM 22 which includes inlet 38, forming vessel 40 (e.g., isopipe), and pull roll assembly 42.
As shown, molten glass 28 flows from downcomer 36 into inlet 38 which leads to forming vessel 40 which is typically made from a ceramic or a glass-ceramic refractory material. Forming vessel 40 includes opening 44 that receives molten glass 28 which flows into trough 46 and then overflows and runs down two lengthwise sides 48 (only one side shown) before fusing together at what is known as root 50. Root 50 is where the two lengthwise sides 48 come together and where the two overflow walls of molten glass 28 rejoin (e.g., refuse) to form glass sheet 12 that is then drawn downward by pull roll assembly 42. The glass sheet cools as it is drawn, transitioning from a molten state at the root, to a visco-elastic state and finally to an elastic state. Pull roll assembly 42 delivers drawn glass sheet 12 which, at the bottom of the isopipe is substantially flat, but which later in the process may develop a slightly bowed or curved shape across the width and/or length of the glass sheet 12. This bowed shape may remain in glass sheet 12 all the way to TAM 24. Continuous beads are formed along the outer periphery of first and second sides 63, 64 of the glass due to the pull rollers contacting the glass. A quality region of the glass is the major surfaces of the glass between the beads, whereas non-quality regions are the regions from the beads to the outer edges at the first and second sides 63, 64 of the sheet.
TAM 24 comprises a laser-mechanical scoring device (or a mechanical scoring device) 52 and nosing device 54 that are used to score the drawn glass sheet, while the TAM moves vertically at the same rate as the glass sheet, so it can then be separated into distinct pieces of glass sheets 56. TAM 24 is located in the elastic region of the sheet in an area referred to herein as a bottom of the draw 58. TAM 24 operates in cycles, the cycle beginning at the first side 63 of the glass at a location that is above the location where the glass will be bent and separated. An optical head and quenching nozzle assembly mounted to the TAM move along the score line from first side 63 toward second side 64 of the glass, while the glass and the TAM continue to travel vertically downward at the same rate. The TAM then reaches the end of its stroke at second side 64 once the laser scoring and quenching processes are completed. The glass bending is carried out along the score line and the robotic equipment located near but below the score line at this point of downward travel of the sheet, separates an individual glass sheet from the continuous sheet. The TAM moves upward, returning to the beginning of the stroke at first side 63 of the glass.
Nosing, pressing, ironing caused by scoring and separating processes cause motion in the glass sheet which in turn contributes to the creation of stress variation within the glass sheet. Sheet motion at the bottom of the draw is mainly driven by sheet scoring and separation processes. Depending on the bottom of the draw setup, robot tooling can also introduce sheet motion. Post separation sheet dangling can be another source of sheet motion if nosing retraction is not controlled properly. On the other hand, over constraint of the sheet by fixed rollers can cause sheet breakage and sheet crackout during the scoring process. Sheet motion produced by any of the above mechanisms, or any other mechanism, can propagate upward into the visco-elastic region of the glass sheet, and becomes especially troublesome in the region where the sheet transitions from a visco-elastic state to an elastic state. Here, stresses caused by movement of the sheet can be frozen into the sheet, and manifest later as, for example, shape changes when the sheet is separated or otherwise further processed.